Although there is increasing concern about inappropriate physician prescribing and how to devise programs to improve drug therapy decisions, little research has been published documenting the reasons for such misprescribing. We analyzed the motivations reported by 141 physicians who were part of a large multi-state randomized controlled trial of 'academic detailing.' The physicians were identified from state Medicaid prescribing records as moderate to high prescribers of cerebral or peripheral vasodilators, propoxyphene, or cephalexin, and were visited by clinical pharmacists serving as outreach educators in a medical school-based prescribing improvement program. Physicians' motivations for their prescribing patterns were discussed in an informal, interactive manner; all responses were recorded in detail by the pharmacists immediately following each visit. Of the 110 responses elicited, the most common reason offered by physicians for use of these medications was patient demand (51 statements, or 46%). Physicians also frequently attributed their prescribing of these drugs to intentional use of placebo effect (24%). An equally common reason was prescribers' assertion that their own clinical experience indicated that these drugs were actually therapies of choice in the conditions presented (26%), despite evidence from the research literature that this was not the case. Such indications included the use of the 'vasodilators' for senile dementia or peripheral vascular disease, cephalexin for viral upper respiratory infections, and propoxyphene instead of acetaminophen or aspirin for mild pain. Greater attention must be paid to physicians' attitudes and motivations concerning suboptimal prescribing if programs are to succeed in replacing these practices with more rational clinical decision-making.